Given that Risako just straight up admitted she lied about a bunch of stuff, I would not put much weight in what she says--but I'm not Kaoru. *rolls eyes*
This is an interesting difference between actual people in real life and characters as plot functions in fiction.
In real life, once someone is revealed to have been lying about something important like this, it’s rational to be distrustful of any such things they subsequently say.
But in this fiction, readers have known for a long time that Reiichi and Risako have been lying; we just haven’t known about exactly what until it was revealed in the latest chapters.
Once Kaoru confronts Reiichi (at long last), his demeanor doesn’t suggest that he is trying to minimize what he had done by claiming that there was no physical cheating. Similarly, Risako doesn’t appear to be trying to wriggle out of a tight spot—on the contrary, she seems to revel in revealing her previous manipulation of the situation, and so far from being distressed, she seems somewhat bemused by her own interest in Kaoru.
Basically, nothing would be accomplished for the story by the further revelation that the affair between Reiichi and Risako was physical rather than just emotional. Kaoru knows the truth about Reiichi’s feelings for her and about the shaky foundation of her marriage. What she does next is unlikely to be affected by finding out that Risako was a double liar rather than just a liar.